Add workaround for cling errors when mixing C++ functors/LoKi

This is a workaround for the issue that produces lots of stderr output in tests such as this one of configurations that use both new C++ functors and LoKi. The underlying issue is described in gaudi/Gaudi#85.

This is clearly just a workaround until there is a better solution, but we should merge it in the meantime.

Please test/merge when you have a chance @cattanem, I don't think there will be any adverse interaction with !1678 (merged) or any other open MRs.

cc: @rmatev @clemenci